The present invention relates to an integrated reception system of great length, for sensing acoustic waves, with sensitive piezoelectric elements distributed along its length, and to a process for manufacturing such a reception system.
The reception system according to the invention finds use, for example, in the field of oceanography or of marine seismic prospecting.
Acoustic reception systems such as those used in marine seismic prospecting, for example, often comprise a very large number of seismic sensing units, such as hydrophones for example, which are distributed at regular intervals all along a supple sheath of very great length or seismic streamer, set up by interconnecting a plurality of sections. The seismic streamer is towed behind a ship moving along path over which the seismic profiling is to be explored, and successive cycles of emission of acoustic signals and of reception of the signals reflected by the subsoil discontinuities in response to the emitted signals are achieved. In modern seismic streamers, the signals picked up by the seismic sensors during each cycle are collected by acquisition devices arranged in housings inserted between the successive streamer sections, sampled, digitized, and stored. The different acquisition devices are connected by common transmission channels, laid all along the streamer, with a central recording laboratory located on the ship. One or several channels are used for transmitting coded orders addressed to the different devices by the central laboratory on the ship.
One or several channels are used for transmitting towards the laboratory the signals emitted by the different devices in response to the received orders. At the end of each emission-reception cycle, the different acquisition devices receive orders to sequentially transmit to the laboratory the data they have stored. Such seismic streamers are, for example, described in U.S. Pat. Nos. 4,398,271 and 4,787,069 and in French Patent Applications FR, 2,653,898 and 2,654,220, all assigned to the same assignee as the present invention.
The sensors arranged along the seismic streamers can be, for example, piezoelectric transducers generally comprising one or several sensitive elements interconnected in series and/or in parallel, each one consisting of a substrate having piezoelectric properties and two electrodes or armatures arranged on either side. The substrate can, for example, be provided in the form of disks made of fritted ceramic with relatively small sizes, included in a rigid housing provided with one or several faces transparent to acoustic waves. The housing is filled with air or with a damping material which absorbs acoustic waves and which can withstand without any distortion considerable variations in the hydrostatic pressure exerted externally on the housings. Such sensors are described, for example, in U.S. Pat. Nos. 3,763,464 and 4,996,675, both assigned to the same assignee as the present invention.
Acoustic sensors of relatively great length without rigid housings are also well-known. They can be sensitive elements of tubular form, such as those described in U.S. Pat. Nos. 3,371,311 and 3,775,737, both assigned to the same assignee as the present invention.
They may also be sensitive elements in the form of supple bands made from materials which are given piezoelectric properties by a treatment. Materials such as PVDF (polyvinylidene fluoride), polyethylene, PTFE (polytetrafluoroethylene), etc, are for example used, or else copolymers which have the property of crystallyzing directly in a polarizable form.
Such sensitive piezoelectric elements are, for example, described in the U.S. Pat. Nos. 4,768,173, 4,810,913, 4,918,666 and 4,984,222, all assigned to the same assignee as the present invention.
Whatever the type of the sensor may be, achieving a reception system suitable for seismic prospecting, for example, is generally very complex. This is due to the large number of sensors to be arranged all along and in the space delimited by the outer protective sheath, as well as to the very large number of electric connections to be carried out in order to connect through conducting wires the different sensors either directly to the recording laboratory or by means of the acquisition devices which are most often inserted between the successive sections of the reception system.